1. Field of the Invention
The present invention relates to a process for the preparation of a gel which is used as a cooling medium, and more particularly to a process for the preparation of a gel having superior properties over those of the known similar gel materials when used as a cooling medium.
2. Prior Art
A variety of gel compositions have hitherto been proposed to be used as cooling media and some of them are applied for practical uses. Such a gel is referred to as a cooling gel, chilling gel, coldness-keeping gel, coldness-keeping heat transfer medium, colloidal coolant, coldness-keeping means, coolant composition or Ice-non (Trade Name), i.e. ice pillow. However, these known coolant gels are not satisfactory in properties or involve troublesome or difficult steps in preparation, as will be summarized in items (1) to (9) below.
(1) The easiest and well-known process for the preparation of a water-containing coolant gel is the preparation of agar gel. This process comprises the step of dissolving agar in boiling water or hot water of 80.degree. to 94.degree. C. to obtain a solution containing not less than 0.1 wt %, for example 1 to 10 wt % of agar and the step of allowing the solution to be cooled to the room temperature, whereby a water-containing gel (hydrogel) having a water content of about 90 to 99 wt % is easily prepared. However, when the thus prepared agar gel is used as a substituting material for ice contained in an ice pillow or an ice bag after being cooled in an ice box of a refrigerator, the agar gel is poor in elasticity and too crumbly to provide a user with a comfortable touch or feeling but gives a strange and uncomfortable feeling with additional disadvantage of collapse.
(2) Although .kappa.-carrageenan including a structure similar to agar, such as D-galactose type structure, 3,6-anhydrogalactose type structure or a partial sulfuric ester type structure of polygalactose, forms readily a hydrogel containing about 90 to 97 wt % of water, this gel is too soft and crumbly. It has been found and emphasized that the mechanical strength of this gel may be improved by immersing the same into an aqueous solution of potassium chloride or calcium chloride by many persons. However, the product obtained by such an after-treatment is too crumbly for practical use, as well as the agar gel.
It is also well-known in the art to combine the carrageenan with a Locust Bean Gum. However, not much effect can be expected by this combination.
(3) It has also been well-known that another type hydrogel having a water content of 97 to 98 wt % can be obtained from an aqueous solution of a sodium salt of alginic acid including a D-manuronic acid type structure or an L-gluronic acid type structure by dropwisely adding to or immersing in an aqueous solution of a water-soluble salt of calcium, barium, zinc, copper, iron, aluminium or nickel or another acidic solution having a pH value of 2.5 to 5. However, such a hydrogel is, as is wellknown, easily to be broken and crumbly with additional disadvantage of displeasing odor.
(4) The known gelatine gel merely exhibits a soft jelly-like appearance at a concentration of gelatine of 2 to 15 wt %. However, although a gel obtainable by using a solution containing more than 15 wt %, particularly more than 30 wt %, of gelatine forms a strong mass, this gel is rigid and lacks elasticity with additional disadvantages that the water-content thereof is low and that it generates glue-like odor.
(5) KONNYAKU (devil's tongue) including a D-mannose structure and a D-glucose structure is the most advantageous amongst a number of natural gels, including gelatine, bean-curd, starch paste, agar, alginic acid, curdlan, carrageenan, furcellaran and pectin, in that it resists deformation and has sufficient elasticity and resiliency and in that a gel having very high water content of up to 97 wt % may be prepared therefrom. However, a gel prepared therefrom tends to be disintegrated rapidly after being repeatedly used as a coolant gel. In detail, although KONNYAKU is relatively stable and excellent in heat resistant property as far as it is immersed into an aqueous alkaline solution containing calcium ions, the gel is changed to a pasty form as the separation and maldistribution of lime are accelerated during the repeated cycles of cooling (freezing) and melting.
(6) It has been proposed to use an iron salt, chromium salt, aluminium salt, lead salt, barium salt or tin salt of carboxymethyl cellulose (sodium cellulose glycolate, hereinafter referred to as CMC). However, a gel obtainable from any of these materials is a soft and weak paste-like gel having a structure which is susceptible to disintegration (In this connection, reference should be made to Japanese Patent Publication No. 11210/1970). Although the gel obtained by the reaction between borax (sodium tetraborate decahydrate) and starch or CMC has been widely known, this gel is soft and weak, as well.
(7) It has been already well-known that gelation occurs instantaneously when boric acid (or an aqueous solution of boric acid) or borax (or an aqueous solution of borax) is added to an aqueous solution of polyvinyl alcohol. However, the gel thus obtained is so soft and weak that it is readily torn to pieces only by pinching by fingers. Another disadvantage of this gel is the syneresis phenomenon, i.e. separation of contained water, during the repeated uses (See Japanese Patent Publication No. 11210/1970).
In order to avoid hardening of a gel of polyvinyl alcohol-borax system due to congeal of water content in the gel when the gel is cooled in an ice-box of a cooling box or refrigerator, it has been proposed a method in which an aqueous solution of polyvinyl alcohol is added with any of monohydric alcohols, polyhydric alcohols, glucose or sucrose followed by the addition of borax to be gelatinized. However, a gel prepared in accordance with this proposal becomes weaker with additional disadvantage that syneresis appears during the repeated uses (See Japanese Patent Publication No. 19602/1971).
(8) Other various proposals have been made to form gels from polyvinyl alcohol, including gelation by the use of phenolic compounds, such as phenol, naphtol or Congo Red, amino compounds or metals, such as titanium, chromium or zirconium. However, all of the gels obtained according to the prior proposals have disadvantages similar to those of the aforementioned polyvinyl alcohol-borax system gel (See Journal of the Chemical Society of Japan, 72, 1058 (1951) and Japanese Patent Publication Nos. 9523/1965 and 23204/1965).
(9) Also, gelation of polyvinyl alcohol by the use of a cross-linking agent or copolymerizable additive, such as aldehydes, dialdehydes, unsaturated nitriles, diisocyanates, trimethylolmelamine, epichlorohydrin, bis(.beta.-hydroxyethyl)sulfone, polyacrylic acid, dimethylol urea or maleic acid anhydride, has been well-known in the art. However, notwithstanding the additional treatment using an additional chemical agent, it was difficult to obtain a strong gel having high water content (In this connection, reference should be made to Textile Res. J., (3), 189 (1962) and the specification of British Patent No. 742,900 (1958)).
Many proposals have been previously made in connection with the method of processing or treating a gel obtained from polyvinyl alcohol. However, these previously-made proposals have disadvantages in operation or in properties of the resultant product, as will be summarized in items (1) to (4) below.
(1) Although a wet or dried membrane may be obtained by drying an aqueous solution of polyvinyl alcohol in air, such a membrane is inferior in water-proof property to form a soft and weak film having no integrity when immersed in water. Accordingly, such a membrane may be used for only limited applications (See Japanese Patent Publication No. 9523/1965).
(2) The properties of a membrane obtained by a method comprising the steps of adding an acid to an aqueous suspension containing polyvinyl alcohol and tetraethyl silicate and then drying in air, are similar to those of the membrane obtained by the method set forth in (1) above. As a modification of the method, there has been proposed a method wherein an aqueous suspension added with an acid is freeze-dried. However, the membrane prepared by the modified method becomes weaker to such an extent that it becomes scarcely possible to mold the same (In this connection, reference should be made to Japanese Patent Publication Nos. 11311/1980 and 30358/1980).
(3) A gelation method wherein an aqueous solution of polyvinyl alcohol is exposed to irradiation of Cobalt 60 (gamma-ray), is also well-known. A special equipment, i.e. an equipment for irradiating radioactive rays, is required in practice of this known method, and additionally the cost for such an irradiation is expensive. Yet, a gel obtained in accordance with this method is often so weak that a further hardening treatment or a secondary hardening step is indispensable. Accordingly, a gel obtained by this method cannot be applied for general uses other than special use in which a highly viscous liquid or soft gel is desired, for example, for use as an artificial vitreous body for filling an intraocular cavity (In this connection, reference should be made to J. Material Sci., 1974, 1815 and Japanese Patent Provisional Publication No. 55647/1975).
(4) It has been well-known since a long time ago that a gel is formed after an aqueous solution of polyvinyl alcohol is stored for one day to one week. However, such a gel is as weak as the agar gel, and may be dissolved again merely by agitating vigorously, by agitating after adding with water or by heating at some extent.